In the present invention, a substrate made by α-alumina (Al2O3) single crystal (hereinafter, it is called as sapphire) is called as sapphire substrate, and a substrate made by polycrystalline alumina (Al2O3) is called as polycrystalline alumina substrate. The sapphire substrate and the polycrystalline alumina substrate are both called as alumina substrate.
A crystalline layer made of a group III nitride semiconductor such as gallium nitride (GaN), aluminum nitride (AlN) or aluminum gallium nitride (AlGaN) or the like is noted as a functional layer constituting a light emitting device and a power transistor of a light emitting diode or a laser diode or the like which emit a short-wavelength light in the range of blue band to ultraviolet band. Further, AlN is also a material which can be expected to be used as a heat dissipation material making use of the high thermal conductivity.
As the forming method of these crystalline layers, a method is proposed in which multi-layered semiconductor thin-film layers are deposited using vapor deposition methods such as molecular beam epitaxy method or metal organic chemical vapor deposition method or the like on a substrate of such as sapphire or SiC single crystal or the like. Especially, a sapphire substrate is an excellent substrate material from the viewpoint of size, supplying capability and cost. However, there is some discrepancy between the lattice constants or the thermal expansion coefficients of the substrate material and the semiconductor thin-film layer because of the different constituent elements, composition ratios or crystalline structures. Due to the discrepancy, internal stress will be produced during the forming process of the semiconductor thin-film layer. As a result, defects or deformation with high density will be introduced to bring decrease in the energy efficiency of the semiconductor element, shortening in the lifetime of the element, inferiority in the property and reduction of yield due to cracking.
Thus, substrates with same materials which are excellent in lattice-match are discussed. For example, respect to the semiconductor thin-film layer of AlGaN containing large amount of Al, AlN single crystal is discussed to be used as the substrate. That is, sublimation methods on the substrates of sapphire or SiC single crystal, vapor deposition methods such as hydride vapor deposition method (HVPE) or the like, or methods in which AlN crystal is made by flux method and a semiconductor thin-film layer of AlGaN is formed on the AlN single crystal. In this case, in order to eliminate the influence of the substrate of sapphire or SiC single crystal or the like which is the base, it is desired to eliminate the sapphire or the SiC single crystal or the like by milling to obtain an independent substrate of AlN single crystal and then deposit a semiconductor thin-film layer of AlGaN on the independent substrate. In order to obtain an independent substrate, the AlN single crystal should be grown to a thickness of 100 μm or more. However, due to the growth on different substrates, internal distortions are accumulated; defects, crackings or warping are included. As a result, influence is brought to the semiconductor thin-film layer of AlGaN which is deposited on the substrate and a semiconductor thin-film layer of AlGaN cannot be formed with sufficient quality in production at last.
As the countermeasure, a method of further growing an AlN single crystal on the independent substrate is proposed. By this method, the quality is expected to be improved; but the process is complex so that the cost is increased. Thus, there is a problem that the utility value in the industry is decreased.
On the other hand, a method is proposed in which AlN single crystal is formed on the substrate of sapphire or SiC single crystal or the like after substances or gaps with different properties from the AlN single crystal are sandwiched in a layer shape and/or a region shape. In such a structure, the internal stress can be suppressed and defect, warping, cracking or deformation can be decreased, or it will be easy to fabricate an independent substrate.
In Patent Document 1, a method is disclosed that a GaN or AlN single crystal is grown after a metal film containing titanium or vanadium is formed on the substrate with a moth-eaten appearance. In this way, GaN or AlN is grown from the moth-eaten part, and stress is released in the part where the metal film is formed.
In Patent Document 2, a method is disclosed in which a part of a substrate surface is decomposed by a thermal treatment in a reducing gas and ammonia gas after an AlN layer is formed on the sapphire substrate, and then gaps are formed at the interface between the substrate and the AlN layer. In the case that the AlN single crystal is grown after the treatment has been carried out, stress strain caused by lattice mismatch can be relaxed due to the void. In addition, when an excessive stress is produced, an independent substrate can be brought out by spontaneous peeling.